The invention relates to a method of connecting two or more elongate connection members, such as risers or tendons, between the seabed and a floating vessel. The invention also relates to a vessel for carrying out this method and to a vessel carrying a connector suspended from the vessel by at least two spaced apart suspension members for relative displacement of the connector with respect to the vessel, one or more elongate connection members, such as risers or tendons, being attached with one end to the connector and with their other end to the seabed.
From U.S. Pat. No. 4,272,059 a riser tensioning system is known wherein a riser, such as a drilling riser, is at its upper end provided with a tension ring which is connected via cables to sheaves on the drilling vessel. The sheaves are mounted on the free end of piston rods of hydraulic cylinders, the second end of the cables is being attached to the vessel. Upon heave, roll or pitch of the vessel the tensional forces on the riser are maintained generally constant by movement of the piston rods against the hydraulic pressure in the cylinders. This system has as a disadvantage that it carries only a single riser and that tensional forces exerted on the riser will vary with the buoyancy of the vessel. In order to obtain a relatively large stroke of the cylinders, these cylinders should be relatively long and therefore take up a lot of space, which in view of the moving nature of the cylinders cannot be effectively used. Furthermore, the hydraulic system is relatively complex.
From U.S. Pat. No. 3,681,928 a barge supporting a drilling rig is known, in which a platform is movably suspended from two mounting arms above deck level of the barge. The platform is connected to the seabed via two parallel tendons which pass through openings in the platform and through a central well in the barge. By this construction the platform remains in a horizontal position and at a constant height above the seabed when the vessel moves vertically due to wave motion. Under the influence of the dependant counterweights, the tendons are kept taut.
It is an object of the present invention to provide a method and a vessel for attaching multiple risers or tendons to the seabed and to the vessel. It is especially an object of the present invention to provide a method of drilling a subsea hydrocarbon well and attaching multiple risers with one end to the wellhead and with their other end to the vessel. Thereto the method is characterised in that it comprises the steps of:
a. attaching one or more connection-members to the connector,
b. increasing the tensioning force on the tensioning means, or vice versa, and
c. repeating steps a and b until the connection members are installed between the vessel and the seabed.
The present invention is based on the insight that a constant tensioning force can be easily maintained on the tendons or risers when they are being connected to the wellhead one after the other, by a stepwise increase of the tensioning force of the tensioning means. Each time after a drilling operation, one or more additional risers are attached to the connector. In case of hydraulic tensioning means, it is envisaged that the oil pressure in the hydraulic system or the air pressure in case of a pneumatic system, is stepwise increased when the number of risers and tendons attached to the connector increases. In case of the connector being suspended from a cable or rod, the tensioning force can be increased in a stepwise manner by adding extra tensioning weights or buoyancy members to the second end of the cable or rod. The increase in the tensioning force according to step b above can be carried out either prior to or after connection of additional connection members to the connector.
The method of the present invention of stepwise increase of the tensioning force is especially suitable for drilling methods in deep water. In this case the risers may have a length of 1000 meters or more and may have a weight of between 40 and 60 tons in submerged conditions. Each time one or more risers are installed, the force exerted by the connector of the present invention is increased a little over 40-60 tons, or a multiplicity thereof corresponding to the number of risers or tendons that are added, such that a substantially constant tension is maintained in the risers or tendons at all times. A further advantage is that, depending on the prevailing current, the tensioning force can be varied to compensate for the current factor and prevent the risers from buckling.
In one embodiment, the connector may be formed by a deck structure suspended from two or more oppositely located pivoting arms, which on their free ends are provided with connection means for attaching a multiplicity of separate weight elements to the free end of the arms.
The connector may also be suspended from cables running along sheaves the free end of the cables carrying a counterweight. Additional counterweight elements may be added around the cable to be suspended from the free end to increase the tensioning force. As is used herein, the term xe2x80x9ccablexe2x80x9d comprises ropes, wires, chains, lines, combinations thereof and any equivalent means.
In one preferred embodiment, the cables have a first section extending vertically downward front the vessel to below water level and a second section which extends in a loop back upwards from the first section to a pulling device on the vessel, tensioning weights being comprised on the first and second cable sections. The tensioning force on the connector can be varied by varying the length of the first and second cable sections via the pulling device. When the loop is made large, the tensioning weight will be placed on the first cable section and will act completely on the connector. When the second cable section is shortened, the tensioning weight will be placed along the second cable section such that less weight is dependant from the connector via the first cable section. The tensioning weight may be comprised of clump-weights added to the cable or may be formed by the weight of the cable section itself, which may for instance be formed by a chain.
In another embodiment, the cables are guided along a cable guide means, such as a sheave, and are with one end attached to the seabed. A take-up device is connected to the cables for varying the cable length. The cable may be comprised of an anchor line such as a polyester line, the tension of which can be varied by the take-up device, which can for instance be formed by a winch. In another embodiment the cables may comprise a chain part carrying clump weights. By varying the length of the cable, the clump weights may be lifted from the seabed one after the other, such that the tensioning force on the connector is increased. The take-up device may be comprised of a winch and chain stopper assembly of the type known in the state of the art.
The varying tensioning force on the connector can in another embodiment be exerted by a buoyancy tank which is attached to the free end of cables from which the connector is suspended. The cables may be guided via a sheave to a cable guide means located below keel level, such that the upward buoyancy force acts on the cable. The buoyancy tanks may for instance exert a maximum upwards force of 600 tons each, three buoyancy tanks being attached to the connector. Pneumatic lines may be attached to the buoyancy tanks for ballasting or deballasting the tanks. In another embodiment, the position of the cable-guide means with respect to keel level of the vessel can be varied such that the tensioning force is increased or decreased.
In a further alternative embodiment, the tensioning force on the suspension members can be varied by movement of the pivot arms from which the risers or tendons are suspended, with respect to pivot points and/or by movement of a counterweight along the pivot arms. The counterweight may be moved for instance by means of a rack and pinion construction.
A vessel according to the present invention, which comprises a connector suspended from at least two spaced apart suspension members, is characterised in that a supporting deck is situated above the connector, a first section of a connection member extending from the seabed to the connector and being detachably connected to a second section of the connection member via coupling device, the second section extending to the supporting deck. Preferably the connector carries a blow-out preventor, the supporting deck carrying a drilling rig. The drill string that is attached to the drilling rig, may be disconnected from rig during high seas, when drilling is suspended. The casing is suspended from the blow-out preventor. When the relative motions between the connector and the deck are again within certain limits, the drill string can be reconnected to the drilling rig and drilling may be resumed. In this way it is not necessary to dismantle the total drill string and drilling riser, because the drill string and drilling riser can after disconnection move independently from the drilling rig. In this way drilling down time in stormy conditions is reduced compared to constructions in which the blow-out preventor is situated on the seabed and disconnection of the drilling riser is effected near the seabed, at the position of the blow-out preventor.